LHS 1140b Research Articles

Modeling the astrosphere of LHS 1140. On the differences of 3D magnetohydrodynamic single- and multi-fluid simulations and the consequences for exoplanetary habitability

The cosmic ray (CR) flux, as well as the hydrogen flux into the atmosphere of an exoplanet, can change the composition of said atmosphere. Here, we present the CR and hydrogen flux above the atmosphere. To do so, we study the 3D multi-fluid magentohydrodynamic (MHD) structure of astrospheres. We aim to discuss the shock structure of the stellar wind of LHS 1140 using four different models: hydrodynamic (HD) and ideal MHD single-fluid models, as well as multi-fluid models for both cases, including a neutral hydrogen flow from the interstellar medium (ISM). The CR flux in a multi-fluid model and the ionization rate in an exoplanetary atmosphere are also presented. The astrosphere is modeled using the 3D Cronos code, while the CR flux at LHS 1140b is calculated using both a 1D and a 3D stochastic Galactic CR (GCR) modulation code. Finally, the atmospheric ionization and radiation dose is estimated using the AtRIS code. It is shown that the 3D multi-fluid positions of the termination (TS) differ remarkably from those found in the 3D ideal-single fluid HD case. CR fluxes computed using a 1D approach are completely different from those calculated using the 3D modulation code and show an essentially unmodulated spectrum at the exoplanet in question. Utilizing these spectra, ionization rates and radiation exposure within the atmosphere of LHS 1140 b are derived. It is shown that the multi-fluid MHD TS distances differ remarkably from those of other models, especially from an analytic approach based on ideal single-fluid HD. The TS, astropause, and bow shock distances must be taken from the 3D multi-fluid MHD model to determine the CR fluxes correctly. Moreover, because of the tiny astrosphere, the exoplanet is submerged in the neutral hydrogen flow of the ISM, which will influence the exoplanetary atmosphere. A 3D approach to GCR modulation in astrospheres is also necessary to avoid unrealistic estimates of GCR intensities. Since atmospheric chemistry processes, and with that, the derivation of transmission spectra features and biosignature information, strongly depend on atmospheric ionization, our results highlight that reliable GCR-induced background radiation information is mandatory, particularly for inactive cool stars such as LHS 1140.

TOI-1452 b: SPIRou and TESS Reveal a Super-Earth in a Temperate Orbit Transiting an M4 Dwarf

Exploring the properties of exoplanets near or inside the radius valley provides insight on the transition from the rocky super-Earths to the larger, hydrogen-rich atmosphere mini-Neptunes. Here, we report the discovery of TOI-1452b, a transiting super-Earth (R p = 1.67 ± 0.07 R ⊕) in an 11.1 day temperate orbit (T eq = 326 ± 7 K) around the primary member (H = 10.0, T eff = 3185 ± 50 K) of a nearby visual-binary M dwarf. The transits were first detected by the Transiting Exoplanet Survey Satellite, then successfully isolated between the two 3.″2 companions with ground-based photometry from the Observatoire du Mont-Mégantic and MuSCAT3. The planetary nature of TOI-1452b was established through high-precision velocimetry with the near-infrared SPIRou spectropolarimeter as part of the ongoing SPIRou Legacy Survey. The measured planetary mass (4.8 ± 1.3 M ⊕) and inferred bulk density ( g cm−3) is suggestive of a rocky core surrounded by a volatile-rich envelope. More quantitatively, the mass and radius of TOI-1452b, combined with the stellar abundance of refractory elements (Fe, Mg, and Si) measured by SPIRou, is consistent with a core-mass fraction of 18% ± 6% and a water-mass fraction of %. The water world candidate TOI-1452b is a prime target for future atmospheric characterization with JWST, featuring a transmission spectroscopy metric similar to other well-known temperate small planets such as LHS 1140b and K2-18 b. The system is located near Webb’s northern continuous viewing zone, implying that is can be followed at almost any moment of the year.

INCREASE: An updated model suite to study the INfluence of Cosmic Rays on Exoplanetary AtmoSpherEs

AbstractExoplanets are as diverse as they are fascinating. They vary from ultrahot Jupiter‐like low‐density planets to presumed gas‐ice‐rock mixture worlds such as GJ 1214b or worlds as LHS 1140b, which features twice the Earth's bulk density. Regarding the great diversity of exoplanetary atmospheres, much remains to be explored. For a few selected objects such as GJ1214b, Proxima Centauri b, and the TRAPPIST‐1 planets, the first observations of their atmospheres have already been achieved or are expected in the near future with the launch of the James Webb Space Telescope envisaged in October 2021. However, in order to interpret these observations, model studies of planetary atmospheres that account for various processes—such as atmospheric escape, outgassing, climate, photochemistry, as well as the physics of air showers and the transport of stellar energetic particles and galactic cosmic rays through the stellar astrospheres and planetary magnetic fields—are necessary. Here, we present our model suite INCREASE, a planned extension of the model suite discussed in Herbst, Grenfell, et al. (2019).

Machine learning techniques in studies of the interior structure of rocky exoplanets

Context. Earth-sized exoplanets have been discovered and characterized thanks to new developments in observational techniques, particularly those planets that may have a rocky composition that is comparable to terrestrial planets of the Solar System. Characterizing the interiors of rocky exoplanets is one of the main objectives in investigations of their habitability. Theoretical mass-radius relations are often used as a tool to constrain the internal structure of rocky exoplanets. But one mass-radius curve only represents a single interior structure and a great deal of computation time is required to obtain all possible interior structures that comply with the given mass and radius of a planet. Aims. We apply a machine-learning approach based on mixture density networks (MDNs) to investigate the interiors of rocky exoplanets. We aim to provide a well-trained MDN model to quickly and efficiently predict the interior structure of rocky exoplanets. Methods. We presented a training data set of rocky exoplanets with masses between 0.1 and 10 Earth masses based on three-layer interior models by assuming Earth-like compositions. This data set was then used to train the MDN model to predict the layer thicknesses and core properties of rocky exoplanets, where planetary mass, radius, and water content are inputs to the MDN. The performance of the trained MDN model was investigated in order to discern its predictive ability. Results. The MDN model is found to show good performance in predicting the layer thicknesses and core properties of rocky exoplanets through a comparison with the real solutions obtained by solving the interior models. We also applied the MDN model to the Earth and the super-Earth exoplanet LHS 1140b. The MDN predictions are in good agreement with the interior model solutions within the uncertainties of planetary mass and radius. More importantly, the MDN model takes a much shorter computational time compared to the cost of the interior model calculations, offering a convenient and powerful tool for quickly obtaining information on planetary interiors.

Hubble WFC3 Spectroscopy of the Habitable-zone Super-Earth LHS 1140 b

Abstract Atmospheric characterization of temperate, rocky planets is the holy grail of exoplanet studies. These worlds are at the limits of our capabilities with current instrumentation in transmission spectroscopy and challenge our state-of-the-art statistical techniques. Here we present the transmission spectrum of the temperate super-Earth LHS 1140b using the Hubble Space Telescope (HST). The Wide Field Camera 3 (WFC3) G141 grism data of this habitable-zone (T eq = 235 K) super-Earth (R = 1.7 R ⊕) shows tentative evidence of water. However, the signal-to-noise ratio, and thus the significance of the detection, is low and stellar contamination models can cause modulation over the spectral band probed. We attempt to correct for contamination using these models and find that, while many still lead to evidence for water, some could provide reasonable fits to the data without the need for molecular absorption although most of these cause features in the visible ground-based data which are nonphysical. Future observations with the James Webb Space Telescope would be capable of confirming, or refuting, this atmospheric detection.

Simultaneous Optical Transmission Spectroscopy of a Terrestrial, Habitable-zone Exoplanet with Two Ground-based Multiobject Spectrographs

Abstract Investigating the atmospheres of rocky exoplanets is key to performing comparative planetology between these worlds and the terrestrial planets that reside in the inner solar system. Terrestrial exoplanet atmospheres exhibit weak signals, and attempting to detect them pushes at the boundaries of what is possible for current instrumentation. We focus on the habitable-zone terrestrial exoplanet LHS 1140b. Given its 25-day orbital period and 2 hr transit duration, capturing transits of LHS 1140b is challenging. We observed two transits of this object, approximately 1 yr apart, which yielded four data sets thanks to our simultaneous use of the IMACS and LDSS3C multiobject spectrographs mounted on the twin Magellan telescopes at Las Campanas Observatory. We present a jointly fit white light curve, as well as jointly fit 20 nm wavelength-binned light curves from which we construct a transmission spectrum. Binning the joint white light-curve residuals to 3-minute time bins gives an rms of 145 ppm; binning down to 10-minute time bins gives an rms of 77 ppm. Our median uncertainty in in the 20 nm wavelength bins is 260 ppm, and we achieve an average precision of 1.3× the photon noise when fitting the wavelength-binned light curves with a Gaussian process regression. Our precision on is a factor of four larger than the feature amplitudes of a clear, hydrogen-dominated atmosphere, meaning that we are not able to test realistic models of LHS 1140b’s atmosphere. The techniques and caveats presented here are applicable to the growing sample of terrestrial worlds in the Transiting Exoplanet Survey Satellite era, as well as to the upcoming generation of ground-based giant segmented mirror telescopes.

Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets

Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems, especially the three nearby ones--Proxima b, TRAPPIST-1e, and LHS 1140b. Previous studies suggest that if these planets have surface ocean they would be in an eyeball-like climate state: ice-free in the vicinity of the substellar point and ice-covered in the rest regions. However, an important component of the climate system--sea ice dynamics has not been well studied in previous studies. A fundamental question is: would the open ocean be stable against a globally ice-covered snowball state? Here we show that sea-ice drift cools the ocean's surface when the ice flows to the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin-orbit resonances (such as 3 to 2). These results suggest that sea-ice drift strongly reduces the open ocean area and can significantly impact the habitability of tidally locked planets.

The high-energy radiation environment of the habitable-zone super-Earth LHS 1140b

Context.In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts the chemistry and evolution of planetary atmospheres. LHS 1140b is one of the most interesting exoplanets discovered. It is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs.Aims.In this work, we present the results of the analysis of aSwiftX-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b.Methods.We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV1344−1786Åand NUV1771−2831Åflux obtained using the sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution.Results.No significant variation of the NUV1771−2831Åflux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV1771−2831Åflux <2% with respect to that of the present-day Earth, while the FUV1344−1786Å/NUV1771−2831Åratio is ~100–200 times higher. This represents a lower limit to the true FUV/NUV ratio since the FUV1344−1786Åband does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio.Conclusions.The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability.

Hydrohalite Salt-albedo Feedback Could Cool M-dwarf Planets

Abstract A possible surface type that may form in the environments of M-dwarf planets is sodium chloride dihydrate, or “hydrohalite” (NaCl · 2H2O), which can precipitate in bare sea ice at low temperatures. Unlike salt-free water ice, hydrohalite is highly reflective in the near-infrared, where M-dwarf stars emit strongly, making the effect of the interaction between hydrohalite and the M-dwarf spectral energy distribution necessary to quantify. We carried out the first exploration of the climatic effect of hydrohalite-induced salt-albedo feedback on extrasolar planets, using a three-dimensional global climate model. Under fixed CO2 conditions, rapidly rotating habitable-zone M-dwarf planets receiving 65% or less of the modern solar constant from their host stars exhibit cooler temperatures when an albedo parameterization for hydrohalite is included in climate simulations, compared to simulations without such a parameterization. Differences in global mean surface temperature with and without this parameterization increase as the instellation is lowered, which may increase CO2 build-up requirements for habitable conditions on planets with active carbon cycles. Synchronously rotating habitable-zone M-dwarf planets appear susceptible to salt-albedo feedback at higher levels of instellation (90% or less of the modern solar constant) than planets with Earth-like rotation periods, due to their cooler minimum dayside temperatures. These instellation levels where hydrohalite seems most relevant correspond to several recently discovered potentially habitable M-dwarf planets, including Proxima Centauri b, TRAPPIST-1e, and LHS 1140b, making an albedo parameterization for hydrohalite of immediate importance in future climate simulations.

Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST

Abstract Nine transiting Earth-sized planets have recently been discovered around nearby late-M dwarfs, including the TRAPPIST-1 planets and two planets discovered by the MEarth survey, GJ 1132b and LHS 1140b. These planets are the smallest known planets that may have atmospheres amenable to detection with the James Webb Space Telescope (JWST). We present model thermal emission and transmission spectra for each planet, varying composition and surface pressure of the atmosphere. We base elemental compositions on those of Earth, Titan, and Venus and calculate the molecular compositions assuming chemical equilibrium, which can strongly depend on temperature. Both thermal emission and transmission spectra are sensitive to the atmospheric composition; thermal emission spectra are sensitive to surface pressure and temperature. We predict the observability of each planet’s atmosphere with JWST. GJ 1132b and TRAPPIST-1b are excellent targets for emission spectroscopy with JWST/MIRI, requiring fewer than 10 eclipse observations. Emission photometry for TRAPPIST-1c requires 5–15 eclipses; LHS 1140b and TRAPPIST-1d, TRAPPIST-1e, and TRAPPIST-1f, which could possibly have surface liquid water, may be accessible with photometry. Seven of the nine planets are strong candidates for transmission spectroscopy measurements with JWST, although the number of transits required depends strongly on the planets’ actual masses. Using the measured masses, fewer than 20 transits are required for a 5σ detection of spectral features for GJ 1132b and six of the TRAPPIST-1 planets. Dedicated campaigns to measure the atmospheres of these nine planets will allow us, for the first time, to probe formation and evolution processes of terrestrial planetary atmospheres beyond our solar system.

A temperate rocky super-Earth transiting a nearby cool star.

M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future.